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CXC Newsletter
The High Resolution Camera
Ralph Kraft, Hans Moritz Guenther (SAO), and
Wolfgang Pietsch (MPE)
It has been another quiet, but successful year for
HRC operations. The instrument performs well with
no significant anomalies or instrumental issues. The
HRC continues to be used for a wide variety of astrophysical investigations. In this chapter, we briefly describe two such science studies. First, a collaboration
led by Dr. Wolfgang Pietsch of MPE used the HRC to
study the X-ray state of the nova population in M31
in conjunction with XMM-Newton and optical studies.
They found that there is a strong correlation between
the optical and X-ray properties of these novae. Second, Guenther et al. (2012) used the HRC-I to measure
the X-ray flux from the nearby star β Pic. Combining
the sensitivity of the HRC-I below 0.5 keV with an
XMM-Newton detection, they demonstrated that the
emission originates in the stellar corona, not the dusty
disk, and that the temperature of the gas is ∼ 1.2 MK,
making it the hottest low-mass stellar coronal source
known.
The Chandra HRC detector was used in a joint
program with the XMM-Newton observatory to study
the supersoft X-ray state of optical novae in our neighbor galaxy M31. X-ray studies of novae are vital to
gain a direct access to the physics of the underlying
white dwarf that hosts the nova event. In six dedicated
monitoring seasons from June 2006 to June 2012 (41
total HRC observations with integrated exposure of
750 ks), we discovered the supersoft X-ray counterparts of 38 novae and thereby doubled the number of
such sources known in M31. The Chandra HRC with
its excellent spatial resolution, large field of view and
response at X-ray energies below 0.5 keV was crucial
for detecting nova counterparts in the crowded central
region of M31, where other X-ray observatories suffer
from source confusion. A statistical analysis of the overall sample of 79 X-ray-detected novae in M31 revealed
strong correlations between the optical and X-ray parameters of novae, showing essentially that novae that
decline rapidly in the optical are hot in X-rays, with
short X-ray phases (Henze et al. 2010, 2011, 2013, see
Fig. 1). In addition to the statistical analysis of the nova
sample, several individual novae could be investigated in detail (e.g. novae in globular clusters and novae
showing time variability).
To create a source catalog of the M31 center and
search for time variability of the sources, we included 23 observations from November 1999 to February
2005 (adding about 250 ks exposure time). We detected
318 X-ray sources and created long term light curves
for all of them. We classified sources as highly variable
or outbursting (with subclasses short vs. long outbursts
and activity periods). 129 sources could be classified as
X-ray binaries due to their position in globular clusters
or their strong time variability (see Fig. 2). We detected
seven supernova remnants, one of which is a new candidate, and also resolved the first X-rays from a known
radio supernova remnant (see Hofmann et al. 2013).
In a second study, Guenther et al. used the HRC to
determine the origin of the X-ray emission from β Pic,
a nearby star with a dusty disk. On the main sequence,
there are two different mechanisms that generate stellar
X-ray emission. Massive O and B stars have radiatively driven winds, and because these winds are unstable,
X-ray emitting shocks form. On the other end to the
main sequence are the cool stars that produce X-ray
emission in a magnetically heated corona like our sun.
Stars of spectral type A lie between these regimes and
should be X-ray dark. Yet, some of them have been detected in X-ray observations. One of the most peculiar
of those objects is β Pic, an A5 star at only 19.5 pc. A
previous 70 ks XMM-Newton observation (Hempel et
al. 2005) barely detected β Pic in a very narrow energy
band just around the O VII band. β Pic also has a strong
IR excess due a a massive debris disk surrounding the
star, made up from dust and cometary bodies. There are
at least three scenarios for the origin of this X-ray emission: (i) a stellar corona, (ii) a boundary layer where
mass is accreted from the disk or (iii) charge-exchange
between the stellar wind and the comas of many comets.
A grating spectrum would easily select between
these models, but alas, β Pic is much too faint. So, how
do we get any spectral information for a star that is detected with just a dozen photons in more than 70 ks of
XMM-Newton? The answer is to go to a different band;
scenario (ii) and (iii) should yield significant soft X-ray
emission in the HRC bandpass, unique to Chandra. So,
we observed β Pic for 20 ks with the HRC-I. In scenarios (ii) and (iii) there should have been hundreds of
counts in the HRC-I, but we only saw 17 (see Fig. 3).
From the ratio of count rates in the XMM-Newton observation and our new Chandra/HRC data, we conclude
that β Pic has a 1.2 MK corona, and this X-ray emission
Spring, 2014
is unrelated to its dust disk. Instead, this makes β Pic the
hottest cool star, i.e. the hottest known star with magnetic activity.
References
Guenther et al. 2012, ApJ, 750, 78
Hempel et al. 2005, A&A, 420, 707
Henze et al. 2010, A&A 523, A89
Henze et al. 2011, A&A 533, A52
Henze et al. 2013, A&A in print arXiv: 1312.1241
Hofmann et al. 2013, A&A 555, A65
Fig. 1 — For novae in M31, (a) X-ray turn-on time versus turn-off time, (b) black body temperature versus
X-ray turn-off time, (c) optical decay time versus X-ray turn-on time, (d) expansion velocity versus X-ray
turn-on time (from Henze et al. 2013).
CXC Newsletter
Fig. 3 — HRC-I image of β Pic, spatially binned by a
factor of two. The shade indicates the number of counts
per bin from light gray (1 count) to black (4 counts). The
circle marks the extraction region centered on the optical position of β Pic (Guenther et al. 2012).
Fig. 2 — X-ray luminosity and photon count rate over
modified Julian date: highly variable (source 104), short
outburst (12), long outburst(184), activity period during
outburst (185) (from Hofmann et al. 2013).
Fig. 4 — Ratio of HRC-I counts (full band) and MOS
counts (O VII) as a function of plasma temperature for
an optically thin, collisionally excited thermal plasma
with solar abundances (Guenther et al. 2012).